In the production of industrially used heterogeneous catalysts, two types can be differentiated, namely supported catalysts and massive catalysts. With supported catalysts, the catalytically active material, for example, metal salts or metal oxides are applied to a catalytically inactive support, for example, aluminum oxide, by immersion or impregnation. The form and size of the support, for example, balls or tablets with dimensions of 10 to 50 mm, determine in this case, the shape and size of the finished catalyst. By contrast thereto, the massive catalyst is comprised of a powder mixture of a catalytically active and an inert mass. It has a shape and size determined by the subsequent shaping of the mixture, for example, by extruding or pelletizing.
The invention relates to massive catalysts. They are produced according to the state of the art by precipitation of a metal salt solution, filtration, drying, calcining and subsequent shaping and optionally reduction, e.g. with hydrogen.
The activity, selectivity and life of a catalyst for a given chemical composition depends to a considerable extent upon its physical structure. One understands physical structure to refer to the dispersity, the surface structure and the pore structure. Correspondingly, for many applications, a fine grained catalyst finely divided within the finished shape of the catalyzer, with a high specific surface area, is preferred.
To obtain an especially high dispersity and surface area of the massive catalyst, it is known in the art to precipitate the catalytically active metal salts together with catalytically inert components. Such catalysts are not supported catalysts but rather are considered massive catalyst which are produced by a so-called "mixed precipitation".
An example of such mixed precipitation is found in DE 39 30 298 A1. Here, a massive copper-zinc-silicate catalyst is produced by addition of a solution of copper nitrate and zinc nitrate to a solution of sodium silicate with vigorous agitation.
It is important with such mixed precipitation that there be a higher solubility product of the active component than that of the inert component at the given pH value. In this case, the inert component precipitates initially in a fine particle size, typically about 500 nm, upon which the smaller particles of the active component can grow. The larger inert particles with the smaller active particles deposited thereon, enable a fine distribution of the active component in the massive catalyst. These particles have, in addition, a high mechanical stability so that the particles of the active component cannot be readily separated from the inert component under customary conditions.
On the contrary, what is unwanted is the opposite ratio of the solubility products. In this case, initially the active components precipitate in the form of relatively large particles upon which the inert fine grained particles deposit. On the one hand, the catalytically effective surface is partly covered by the inert particles. On the other, there is a further drawback in the experimentally determinable reduction in the adhesion of the active particles on the inert particles in the mixed particles mass.
The mixed precipitation under the above mentioned desirable conditions, has several drawbacks. One is the fact that a satisfactory difference of the solubility products of active and inert components corresponding to the aforedescribed requirements only occurs in a limited range of pH values. However, the pH value changes during the precipitation so that, in many cases, only at the beginning of the precipitation, but not toward the end of precipitation, is there a sufficient difference of the solubility products of the two components.
It frequently happens that toward the end of the precipitation the solubility products of the two components become sufficiently close that both components precipitate practically simultaneously and a growth of the active component on the previous precipitated support crystallites no longer occurs.
A further disadvantage resides in the limitation of the components which can be used to those with suitable differences in the solubility products. Thus, the solubility product of the inert component must be less than 10.sup.-9. A mixed precipitation with titanium dioxide as the inert component, which has a solubility product of 10.sup.-5, and with copper as an active component, is thus not possible.
Apart from precipitation or mixed precipitation, there are in the art alternative process for producing heterogeneous massive catalysts. The requisite fine distribution of the active component is here not reached in a physical-chemical manner but rather by mechanical comminution.
Thus heterogeneous catalysts on the basis of monocrystalline alloys are produced by mechanical alloying (WO 90/09846 A1). The copper and nickel containing catalysts contain as inert components, silicon or silicon dioxide. A special advantage of these catalysts resides in their waterfree production process. For production, the corresponding metal powders are milled with high energy in a ball mill. In addition to the milling, there is a welding of the metal powder particles to one another. Responsible for the resulting alloys is a diffusion controlled solid body reaction between the thin rolled out layers. One obtains crystallite sizes below 10 nm.
The production of catalysts containing inert components corresponding to the process described at the outset, are described in addition in DE 43 08 120 A1 and WO 94/15708 A1. DE 42 09 292 A1 describes a process for processing commercially useful catalysts which is practiced in a similar manner.
According to DE 43 08 120 A1, a catalyst is produced by mixing together the starting materials forming the solid body and the catalyst forming oxidic substances, which can be apart from copper oxide or another metal oxide, at least one further metal oxide, for example, aluminum oxide, and simultaneously or subsequently comminuting the oxides to a particle size less than 10 .mu.m mechanically. Preferably the d.sub.50 value of the comminuted starting materials lies between 0.1 .mu.m and 1 .mu.m.
The comminuted starting materials according to the examples of this publication can be copper oxide and titanium oxide or copper oxide and aluminum oxide, with one being a catalytically active component and the other being a catalytically inert component. According to a preferred embodiment of the known process, the comminution is carried out with introduction of a liquid, especially water. However, separate suspensions of the catalytically active and catalytically inert components are not produced but both components are milled together with one another simultaneously. A given ratio of the particle sizes of the catalytically active and the catalytically inert components cannot be maintained in this manner. From DE 43 08 120 A1 as well as from WO 94/15708A1, numerical values of this particle size ratio cannot be deduced. If, however, a certain particle size ratio of the active and inert components is not obtained, there are the following drawbacks whose origins have been deduced by me.
The specific surface area (SA) in m.sup.2 /g of ultrafine particles in the size range of 1000 mm and less depends upon the specific gravity D(in g/ml) and the mean particle diameter d.sub.50 (in nm) and is given by the formula SA=6000/(D.multidot.d.sub.50). The specific area can have the following values at d.sub.50 =500 nm as given for several typical catalyst components:
______________________________________ TiO.sub.2 3.0 m.sup.2 /g CuO 1.9 m.sup.2 /g Cr.sub.2 O.sub.3 2.3 m.sup.2 /g Al.sub.2 O.sub.3 3.0 m.sup.2 /g SiO.sub.2 5.5 m.sup.2 /g ZnO 2.1 m.sup.2 /g ______________________________________
The particles of the specifically lighter inert components have thus a greater specific area than the particles of the specifically heavier active components for a given size range. A common milling of the active and the inert components to a common particle size thus gives rise to a greater specific surface area of the inert particles relative to the active particles. A coating of the surface of the active particles with the inert particles and thus a reduction of the catalyst activity is the consequence.
Apart from this drawback, these two last mentioned publications are characterized also in that during the comminution a very high energy is applied to the starting materials. In the examples, the energy input during the milling is 30 or 150 kW per liter of reactor volume. In the preferred embodiment of the known process the energy density referred to is up to 500 kW per liter of reactor volume. With such energy inputs, there is not only a mechanical comminution but, as has been described also in DE 43 08 120 A1, a solid body reaction similar to that in the case of the already described mechanical alloying (WO 90/09846). Milling with such high energy inputs requires cooling devices which are of high apparatus cost. In the comminution of the starting materials, temperatures up to 400.degree. C. can also arise.
In the regenerating process according to DE 42 09 292 A1, the catalysts used are also mechanically comminuted to a particle size smaller than 10 .mu.m, preferably to reach a particle size between 0.1 .mu.m and 4 .mu.m. During the comminution, the catalyst is impacted with a mechanical energy with an energy density of up to 500 kW per liter of the reactor volume so as to carry out a solid body reaction. The reprocessed catalyst can be used anew to carry out reactions.
In this process, the active and inert components are comminuted together and to the same particle size so that for the reasons already mentioned, a reduced catalyst activity results.